The present invention relates to a camera lens barrel in a video camera or the like. More particularly, this invention relates to a driving device for a zoom lens in a camera lens barrel of an inner focus type having a four-group zoom lens system with an autofocus function and a motor-driven zoom function.
In a video camera having an autofocus function and a zoom function, it is increasingly demanded to have a more compact and handier construction as well as an improvement in performance. To this end, various attempts have been made to improve upon a camera lens barrel in the video camera (e.g., as shown Japanese Patent Laid-open Publication No. 1-196011 and Japanese Patent Application No. 2-130580 entitled "Camera Lens Barrel" filed by the present assignee).
As a lens system to be accommodated in the camera lens barrel a zoom lens system consisting of four groups of lenses is known. Such a four-group zoom lens system has two types. One is a front focus type such that a first grouped lens, a second grouped lens and a third grouped lens of the four groups of lenses are adapted to be moved. The other is an inner focus type such that a fourth grouped lens of the four groups of lenses has a focusing function, a correcting function and an imaging function, and that the second grouped lens and the fourth grouped lens are adapted to be moved. The present invention relates particularly to the latter inner focus type of a zoom lens system.
In a conventional camera lens barrel of the inner focus type, the second grouped lens for zooming to provide a variable magnification is moved in a direction of an optical axis along two guide shafts fixed to the lens barrel by using a DC motor, a reduction gear for reducing a rotating speed of the DC motor, and a cam, since a moving quantity of the second grouped lens is large. On the other hand, the fourth grouped lens for imaging is driven by a voice coil motor which is linearly operated.
However, when driving the second grouped lens by using the DC motor, the reduction gear, and the cam as mentioned above, there are many problems such as generation of noise to an electronic circuit in the video camera, generation of vibration in driving, low reliability of a brush of the DC motor, a difficulty in high-speed zooming, and a difficulty in providing a compact construction.